U.S. Pat. No. 6,445,939 describes OCT as a method of measuring a tomographic structure of the lumen of an object with a luminal shape, and an optical probe that is inserted into the lumen of the object and used for the OCT measurement. For the OCT measurement, a graded-index optical fiber connected with a tip end (a distal end) of a single-mode optical fiber functions as a lens, and is configured to have a working distance larger than 1 mm and a spot size smaller than 100 μm. Accordingly, an object having an inner radius larger than 1 mm can be optically measured with a spatial resolution smaller than 100 μm.
In the OCT measurement, light output from a light source is branched into two of illumination light and reference light. An optical probe irradiates an object with the illumination light, guides backward reflection light, which is generated at the object as the result of the irradiation, to an optical detector, and also guides the reference light to the optical detector. Then, the optical detector detects interference light of the backward reflection light and the reference light. An analyzing part analyzes a spectrum of the backward reflection light, and acquires distribution information of a substance in the object as image information.